1. Field of the Invention
The present invention relates generally to electronic ballasts for fluorescent lamps, and relates more particularly to electronic ballast controls with adjustable oscillators.
2. Description of Related Art
Electronic ballasts for fluorescent lighting applications are widely available and well known, particularly those that operate with a switching half-bridge. Such an electronic ballast is illustrated in U.S. Pat. No. 6,008,593 to International Rectifier Corporation. Electronic ballast controls have evolved to include a wide range of functionality and features including power factor correction and fault detection and response circuitry.
A typical electronic ballast that includes a switching half-bridge provides an oscillator that is used to derive the switching signals for the half-bridge to appropriately direct current to various components at particular times to establish desired power flow to the fluorescent lamp. One type of implementation of an electronic ballast using an oscillator involves connecting a voltage controlled oscillator (VCO) into the electronic ballast and driving the VCO with an appropriate signal to modify the switching frequency as desired. For example, in the case of fluorescent lamp dimming applications, the switching frequency of the electronic ballast can be adjusted to obtain particular dimming settings.
The use of a VCO in an electronic ballast entails a number of design challenges that include appropriately providing the input to the VCO to obtain the desired oscillating frequency. In addition, a feedback from the output stage of the electronic ballast is typically desired so that appropriate control for the electronic ballast can be maintained with the VCO. When supplying a VCO in an integrated control solution for an electronic ballast, the VCO can take up a large amount of room relative to the other components in the integrated solution.
It would be desirable to obtain a simple oscillator that can be easily controlled and implemented in a simplistic fashion to provide an oscillator function for switching a half-bridge in an electronic ballast.